1. Field of the Invention
The present invention relates to low frequency radio transmission systems; and more particularly, to a method and system for radiating RF power from a trailing wire antenna.
2. Description of the Prior Art
Low and very low frequencies, such as from 60 to 17 KHz, are presently used for narrow band radio communication over long distances because reliable propagation is available at such frequencies. Since physically large antennas are required to radiate significant power at these frequencies, airborne transmitters utilize a trailing wire antenna that is stored on a reel in the aircraft when the transmitter is not in use, and is unreeled when transmission is required. Such a trailing wire antenna, when fully extended for the LF frequency range may be anywhere from 8,000 to 15,000 feet in length. The antenna wire is energized by applying an RF feed voltage to the wire at the aircraft end; and the impedance seen by the applied RF feed voltage is the high reactance of the aircraft in series with the impedance of the antenna wire that is fed by the RF voltage at the one end. The highest voltage that may be applied between the aircraft and the antenna wire is approximately thirty kilovolts root mean squared (KVRMS) because of corona and/or insulation limitations. The current going into the antenna wire is limited essentially by this high reactance; and thus, the power input to the antenna depends on the input resistance which is relatively low at antenna wire lengths that are considerably less than one-half of the transmitted wavelength (.lambda./2). The highest input resistance occurs when the antenna wire is slightly less than .lambda./2 in length; and such resistance is typically around 1,000 ohms. This permits somewhat more than 100 kilowatts of RF power to enter the wire as limited by the 30 KVRMS feed voltage.
Prior to the present invention, the fixed power level of the transmitter was applied to the antenna wire without exceeding the allowable antenna voltage at a time when the airborne trailing wire antenna was at its optimum length, which was near .lambda./2. Extending the low frequency antenna wire to its optimum length required many minutes because of the long wire length. For example, at the low frequencies mentioned of from 60 to 27 KHz, for example, the extension time varied from approximately 8 to 15 minutes at a normal extension of 1,000 ft./min. Also, in the event that it became impracticable to extend the antenna wire to its full optimum length, or in the event that a portion of such wire became separated from the aircraft, the use of the transmitter became infeasible. Further, particularly in military applications, the delay in the beginning of message transmission may not satisfy mission requirements and also decreases in many instances the survivability of the transmitting aircraft.
In the past, various apparatus and systems have been described for reeling in and out a trailing wire antenna for an airborne radio transmitter together with proposed systems for matching the impedance of the transmission line to the impedance of the transmitter, and for effecting resonance between the transmitter and its trailing wire antenna. For example, U.S. Pat. No. 2,417,191, entitled "Airplane Antenna Automatic Tuning System" appears to propose a system for controlling the effective length of the antenna for effectively radiating energy within the frequency range of the transmitter. This patent describes a tuning element in series between the transmitter and its trailing wire antenna with a differential control circuit that has a connection for adjusting the tuning element of the transmitter to effect resonance between the trailing wire antenna and its transmitter. Also, in U.S. Pat. No. 2,430,173, entitled "Remotely Controlled Antenna Tuning Apparatus" there is described a system for tuning and matching an antenna to its transmitter wherein a terminating dummy impedance is provided that matches the transmission line, and a transformer having a variable inductive coupling for matching the impedance of the antenna to a value substantially equal to the impedance of the dummy impedance. Then the dummy impedance and the transformer are alternately connected to the transmission line, with the antenna length being adjusted in accordance with the input current to the transmission line.
However, as far as is known, the prior art does not disclose an apparatus or system for a very low frequency range high power airborne transmitter having an extendable and retractable trailing wire antenna where the power amplifier means output is varied for maximum power transmission while maintaining the voltage and current at the output of the power amplifier means substantially in phase and within the limitations imposed by the maximum allowable antenna feed voltage for the extended length; while providing means that are responsive to the voltages and currents in the antenna to generate real time control signals for the tuning of the antenna system to provide a stable load impedance for the power amplifier in the face of rapidly varying antenna impedance.